Integrally hinged panels have been fabricated of certain plastic materials such as polypropylene by thinning a section of the part and flexing the part immediately after molding along the thinned section to orient the molecular structure and forming a durable flex portion. Frequently, the flex portion of the hinge is indefinite since the comparatively low flexural modulus allows additional flexing between the mounting points of the integrally hinged panel and its mounting surface. Solutions to this problem have included the addition of reinforcement ribs, metal reinforcement members or increased thickness of panel sections in areas immediate adjacent the flex portion.
It has also been proposed to make hinged articles of which the panels have foam cores with unfoamed surfaces, the foam cored panel surfaces being connected by a hinge of unfoamed material. To form such articles an expandable mold has been filled with a foamable composition containing a thermoplastic polymeric material and a blowing agent and expanding the panel forming portions of the mold while holding a hinge defining mold component to prevent foaming in a hinge portion. This process has required that the hinge forming member of the mold have substantially straight sides in order to avoid surface irregularities at the juncture between the hinge forming member and the panel forming portions of the mold. That is, in the procedure heretofore suggested in which the surface portions of the thermoplastic material in the mold are chilled before expansion of the mold, any hinge forming structure other than a straight sided member would generate solidified sections not capable of coalescing homogenously into the panel sections and would form irregular juncture portions having unsatisfactory appearance and forming lines of stress concentration tending to weaken the article.